Melting of the Greenland ice sheet has hit a rate unmatched in the last 12,000 years and is accelerating, scientists have confirmed.
Research published in Nature today predicts that the Greenland ice sheet will be melting by as much as six times its current rate by the end of the century if we don't get emissions down.
On the flipside, if we can achieve the best-case emissions reduction scenario forecast by the IPCC we can limit its increasing melt rate to around 40 per cent greater than its present rate.
As the earth emerged from the last Ice Age around 11,000 years ago, the Arctic experienced a warm period or thermal maximum between about 10,000 and 7,000 years before present.
Researchers presumed that the rate of melting of the Greenland ice sheet in that periodwas higher than it is today.
Instead, they found that over the last 20 years, the southwestern Greenland ice sheet where this research was focussed, has been losing ice at an rate of about 6,100 billion tonnes per century on average — around 100 billion tonnes more than at its previous historical peak, according to author Jason Briner from the University of Buffalo.
"Our results suggest that yes, this century we will experience ice-loss rates not just similar to those in the past but exceeding those of the past, even under strict carbon emissions scenarios," Professor Briner said.
Worst-case scenario would see 600% increase in melting this century
As well as comparing present-day melting with the past, they looked at how different global greenhouse gas emissions trajectories would impact melting over the coming century.
They modelled the IPCC's best-case emissions scenario, called Representative Concentration Pathway (RCP) 2.6, and the worst-case emissions scenario called RCP 8.5.
Under RCP 2.6, emissions are drastically reduced starting now and we achieve net-negative emissions this century. That is, we get our emissions to zero and also draw greenhouse gases from the atmosphere through technology or by boosting natural sinks like forests and blue carbon.
Under RCP 2.6 we limit global average warming to within 2 degrees Celsius by 2100.
On the other hand, under RCP 8.5 we continue burning fossil fuels as per usual, making no substantial efforts to reduce our greenhouse gas emissions through to 2100.
Under the RCP 2.6 scenario, their models forecast that melting of the southwestern Greenland ice sheet would increase to around 8,800 billion tonnes per century on average by 2100 — about a forty per cent increase on today's rate.
But under the worst-case RCP 8.5 scenario, they forecast the southwestern Greenland ice sheet could be losing up to 35,900 billion tonnes per century — an increase of nearly 600 per cent on today's melting rate.
Although their study area didn't encompass the entire ice sheet, Professor Briner said Greenland tends to melt fairly uniformly.
"Based on reconstructions of ice sheet changes over the past several decades, it has been shown that when the ice sheet loses mass in our study area, it loses mass across its entire surface," he said.
"When rates of ice loss are high across our study area, they are high across Greenland."
The study is an important demonstration of the difference that we can make by cutting emissions, according to David Etheridge from the CSIRO's Climate Science Centre.
"The range of predictions shows a high sensitivity to emissions scenarios with the possibility to limit ice loss with low emissions," Dr Etheridge said.
'We still have time' to slow down sea level rise
Modelling sea level rise was outside the bounds of this study, but the researchers tentatively suggest that the worst-case scenario melting from the southwestern Greenland ice sheet would add around 10 centimetres to sea levels this century.
If that was scaled to the entire ice sheet, that would likely be "doubled or tripled", Professor Briner said. And that's without accounting for any melting from the Antarctic ice sheet.
Research published earlier this month in Nature found that the Antarctic ice sheet will add 1.3 metres to sea level for every degree of warming up to 2 degrees Celsius above pre-industrial levels.
That research found that we have locked in at least two-and-a-half metres of sea level rise from Antarctica, regardless of what happens with our emissions from now on.
But it's the rate at which the melting happens that we have some control over and is the crucial issue, according to Will Steffen from the ANU's Climate Change Institute.
"We can still influence the rate at which Greenland melts and thus the rate that sea level rises. That's the important message," Professor Steffen said.
"We still have time to adapt if we slow it down."
Rather than needing to adapt and shift coastal communities over decades, we can buy ourselves a century or more if we act to get emissions down immediately, Professor Steffen said.
When countries signed up to the Paris Agreement in 2015, the aim was to keep warming to 1.5C.
Given our lag in getting emissions down since then, Professor Steffen said he doesn't think 1.5C is still realistically achievable, but keeping warming below 2C is.
Last week China — the world's biggest polluter — committed to achieving net-zero emissions by 2060.
While he is cautiously optimistic, Professor Steffen said what is equally important is how they get there.
Currently China are responsible for about a quarter of the world's greenhouse gas production, and if they begin rapidly cutting emissions from today right through until 2060, then that is significant, he said.
But if they continue burning fossil fuels and make rapid cuts at the last minute, the damage will already be done.
"We use what's called the carbon budget approach to estimate how much temperature is going to rise — that is the cumulative emissions between now and net-zero [emissions]," Professor Steffen said.
"If [China] get their emissions down really quickly, say by 2040, that's a big difference between whether they coast to 2050 and then cut them over a decade."